Proper heating of lamp filaments helps extend the life of gas discharge lamps. Prior to the ignition of the lamp filaments, a filament heating voltage may warm the lamp filaments and may be necessary to ignite certain types of high impedance gas-discharge lamps, such as T5 fluorescent lamps. Filament heating voltages also maintain lamp filament temperatures if the lamp current gets too low when a lamp is being dimmed.
Ballast circuits are used to power and operate gas-discharge lamps. The ballast circuit may include an inverter with at least one switch device utilized to convert a DC voltage into a pulsed voltage and a resonant circuit that converts the pulsed voltage into the required AC voltage for powering the gas-discharge lamp. A filament heating circuit is coupled to the inverter to transmit the filament heating voltage to the lamp filaments. These filament heating circuits may include filament resonant tanks tuned to receive the filament heating voltage during the pre-heat period and/or when the lamp is being dimmed.
While the filament heating circuits may be designed to provide the appropriate filament heating voltage to the lamp filaments, the electrical components utilized in these filament resonant tanks often have unacceptably large component tolerances. Because a filament resonant tank has such a high Q, this may result in excessively high filament heating voltages.
High impedance gas-discharge lamps, such as T5 lamps, are particularly sensitive to filament heating voltages during lamp operation. Filament heating voltages can cause unbalanced filament currents and over-current problems. Maintaining these filament voltages as low as possible when the lamp is being dimmed reduces these problems.
Prior art circuits do exist for cutting off a filament heating voltage during steady state operation. Unfortunately, these circuits are designed to eliminate the filament heating voltage after the pre-heat period. Thus, these circuits cannot be used during the pre-heat period or when a lamp is being dimmed because they are designed to prevent the filament heating voltage from heating the lamp filaments.
What is needed then is a ballast circuit that is capable of reducing the filament heating voltage below a desired maximum level during the pre-heat period and when the lamp is being dimmed.